1. Field of the Invention
This invention relates to a load driving circuit for driving a load such as a motor, and more particularly to a load driving circuit having a pair of push-pull circuits which are constituted by bipolar power transistors to deal with a peak current of 2 to 3 A or more and driving a load connected between the output terminals of the push-pull circuits.
2. Description of the Related Art
FIG. 1 shows a conventional motor driving circuit for driving a bi-directional motor. The circuit includes first and second push-pull circuits 11 and 12 and driving transistors QP1 and QP2 for driving the push-pull circuits 11 and 12. The first push-pull circuit 11 is constituted by NPN output (power) transistors QN1 and QN2 whose current paths are serially connected between a power source Vcc and a ground terminal GND. Likewise, the second push-pull circuit 12 is constituted by NPN output transistors QN3 and QN4 whose current paths are serially connected between the power source Vcc and the ground terminal GND. The driving transistors QP1 and QP2 are used to control the pair of push-pull circuits 11 and 12 so a to change the polarity of a current to be supplied to a bi-directional motor M and are each constituted by a PNP multi-collector (two-collector) transistor. The emitter of the transistor QP1 is connected to the power source Vcc and the first and second collectors thereof are respectively connected to the bases of the output transistors QN1 and QN4. The emitter of the transistor QP2 is connected to the power source Vcc and the first and second collectors thereof are respectively connected to the bases of the output transistors QN3 and QN2. The bases of the driving transistors QP1 and QP2 are respectively supplied with driving control signals SD1 and SD2 which are set in an inverted relationship. The bi-directional motor M connected between the output terminals of the pair of push-pull circuits 11 and 12 is rotated in a forward or reverse direction according to the levels of the driving control signals SD1 and SD2.
Next, the operation of the motor driving circuit shown in FIG. 1 is explained. When the driving control signal SD1 is set to a low level and the driving control signal SD2 is set to a high level, the transistors QP1 and QP2 are respectively turned on and off, the output transistors QN1 and QN2 of the push-pull circuit 11 are respectively turned on and off, and the output transistors QN3 and QN4 of the push-pull circuit 12 are respectively turned off and on. As a result, a current flows from the power source Vcc into the ground terminal GND via the collector-emitter path of the output transistor QN1, the motor M, and the collector-emitter path of the output transistor QN4. Therefore, a current flows in the motor M in a direction indicated by an arrow A, thereby rotating the motor M in a forward direction. On the other hand, when the driving control signal SD1 is set to a high level and the driving control signal SD2 is set to a low level, the transistors QP2 and QP1 are respectively turned on and off, the output transistors QN1 and QN2 of the push-pull circuit 11 are respectively turned off and on, and the output transistors QN3 and QN4 of the push-pull circuit 12 are respectively turned on and off. As a result, a current flows from the power source Vcc into the ground terminal GND via the collector-emitter path of the output transistor QN3, the motor M, and the collector-emitter path of the output transistor QN2. Therefore, a current flows in the motor M in a direction indicated by an arrow B, thereby rotating the motor M in a reverse direction.
Thus, the motor driving circuit shown in FIG. 1 may change the polarity of a current supplied to the motor M according to the levels of the driving control signals SD1 and SD2 supplied to the bases of the driving transistors QP1 and QP2, thereby making it possible to change the rotation direction of the motor M.
In a case where the levels of the driving control signals SD1 and SD2 are changed in order to change the rotation direction of the motor M, both of the output transistors QN1 and QN2 of the first push-pull circuit 11 or both of the output transistors QN3 and QN4 of the second push-pull circuit 12 may be set in the ON state at the same time if one of the driving transistors QP1 and QP2 is turned off with time delay when the other transistor is turned on. At this time, a large through current flows between the power source Vcc and the ground terminal GND, causing the transistor to be deteriorated or damaged. Particularly, when the driving transistors QP1 and QP2 are formed of a lateral PNP transistor, the transition time from the ON state to the OFF state is long, making the above problem more serious.
In order to prevent the through current, various types of circuits have been proposed in the prior art. As a typical example of the circuit, there is provided a circuit system which supplies the driving control signals SD1 and SD2 to a logic circuit to control the timing of inversion so that one of the driving transistors will not be turned on before the other driving transistor is turned off wherein and all of the output transistors QN1 to QN4 may be set in the OFF state for a preset period of time when the rotation direction of the motor M is changed. There is also known another circuit system in which a circuit having a time constant is connected to the bases of the output transistors QN2 and QN4 on the side of the ground terminal GND so as to prevent the output transistors constituting the push-pull circuit from being set in the ON state at the same time.
However, the logic circuit for preventing the flow of through current requires a large number of elements, increasing the chip size when the motor driving circuit is formed in an integrated circuit configuration. Further, when the impedance of the circuit for giving the time constant to the bases of the output transistors QN2 and QN4 on the side of the ground terminal GND is increased, the number of elements used becomes large, and when the impedance thereof is reduced, a correspondingly large capacitance is required, making it difficult to integrate the circuit and thereby increasing the number of parts to be externally attached.